This invention relates to a method of preparing a self-curing two-component powder liquid bone cement comprising a powder component consisting of particles containing a polymerization catalyst, and a liquid component containing a polymerizable monomer, prepolymer or mixtures thereof.
Various proposals have been made for mixing bone cement under vacuum, but mostly for avoiding inhalation of the toxic monomer vapours developing during preparation of bone cement. No utilizable cement mixture is obtained with all these known methods unless the powder and liquid components are not mechanically stirred and homogenized.
It is known, however, that such mechanical stirring, even under vacuum, produces air inclusions which weaken the finally hardened cement mass. Furthermore the accuracy of the powder liquid ratio cannot be guaranteed with the known methods of bone cement mixing since errors of the operator are always possible.
In the pending but not prepublished International Patent Application No. PCT/EP 85/00227 a method for mixing bone cement under vacuum according to the preamble of claim 1 of the present invention is described which uses normal polymethylmethacrylate (PMMA) beads coated with benzoyl peroxide (BPO) as a polymerization catalyst. Whereas this method overcomes the most essential drawbacks of commercially available bone cements, namely poor mixing, high exotherm and creation of porosities, its disadvantage lies in the fact that the BPO deposited on the surface of the PMMA-beads is quickly washed away when the evacuated powder component is flooded with the liquid component (methylmethacrylate) producing an inhomogeneous gradiant in the BPO concentration and thereby an irregular polymerization process.